Rewritable optical disks can be roughly classified into the magneto-optic type disks and the phase-change type disks. In the phase-change disk, a recording film is reversibly converted between amorphous state and crystalline state by changing irradiation conditions of the laser beams so as to record signals, erase recorded signals and perform reproduction by optically detecting the difference in reflectance between an amorphous state and crystalline state of the recording film. In the GeTeSb phase change system the amorphous state of the recording film is defined as the recording state and the an crystalline state of the recording film is defined as the erasing state. Thus, in the recording stage of the phase-change type disk, one laser beam with a short and high-power pulse is focused onto the phase-change type disk for melting, and the melted area of the film transfers to an amorphous state by rapid cooling. When performing the recorded signals erase, another laser beam with a longer and middle-power pulse is focused to the phase-change film for annealing. The focused area of the phase-change type disk is heated and then switched either from an amorphous state to a crystalline state or from a crystalline state to a crystalline state. The recorded information on the film is consequently erased. Since the reflectance of the phase-change type disk in the crystalline state and the amorphous state is different, a beam with a constant low power is used to detect the intensity difference of the reflected beam. Information recorded on the disc is reproduced.
Since the phase change type disk player detects the reflectance difference between the amorphous and a crystalline state to distinguish the digital signals, it is required to have a higher reflectance contrast and provide a better read signal.
Referring to FIG. 1, a phase-change disk in general includes a protective layer 6, a reflecting layer 5, a second dielectric layer 4, a recording layer 3 and a first dielectric layer 2 which are stacked sequentially on a substrate 1. The first dielectric layer 2 and the second dielectric layer 4 may be made of one of the compounds, or their combination, of at least SiO.sub.2, ZnS, TaO.sub.2, GeO, AlN and Si.sub.3 N.sub.4. The substrate 1 may be made of polymethyl methacrylate (PMMA), polycarbonate (PC) or transparent glass, etc. The reflecting layer 5 may be made of one of the metals, or their alloy, of at least Al, Ag, Au, Ni, Cr, Pt and Pd. The protective layer 6 is made of resin or UV curable plastic material. The recording layer 3 is made of phase-change type material that has the characteristic to switch between crystalline and amorphous states. Most of the phase change type materials are chalcogenide alloy which contains elements, such as O, S, Se, Te and Po. Two phase-change type materials Te.sub.85 Ge.sub.15 and Te.sub.81 Ge.sub.15 S.sub.2 Sb.sub.2 were first disclosed in U.S. Pat. No. 3,530,441 by S. R. Ovshinsky. Other phase-change materials such as GeTe, InSe, InSeTe, InSeTeCo, GeTeSb, GeTeSn etc. were subsequently developed. A chemical composition close to Ge.sub.2 Sb.sub.2.2 Te.sub.5 alloy discovered by the Matsushita Electrical Industries Company was first used as a rewritable medium.
The pseudo-binary alloy GeTe--Sb.sub.2 Te.sub.3 was first disclosed in Izv. Akad. Nauk SSSR Mater. 1(2) pp. 204 (1965) by N. Kh. Abrikosov. In the ternary system of Ge--Sb--Te, three ternary compounds GeSb.sub.4 Te.sub.7, GeSb.sub.2 Te.sub.4 and Ge.sub.2 Sb.sub.2 Te.sub.5 are founded to stand in a row on the pseudo-binary tie-line connecting GeTe and Sb.sub.2 Te.sub.3. Disclosed in J. Appl. Phys. 69(5) pp. 2849 (1991) by Yamada etc., a phase-change film can have a relatively short crystallization time. It is found that a pseudo-binary alloy on the GeTe--Sb.sub.2 Te.sub.3 tie-line can be crystallized within 100 nanosecond, and a shorter crystallizing period is performed with increasing the Sb.sub.2 Te.sub.3 content along the pseudo-binary tie-line. In addition, the critical temperature of crystalline monotonically increases with increasing GeTe content along the pseudo-binary line. In the phase diagram of the pseudo-binary alloy GeTe--Sb.sub.2 Te.sub.3, it tends to crystallize to a metastable FCC (Face-Centered Cubic) structure and then to a stable hexagonal structure.
In U.S. Pat. No. 5,278,011, the pseudo-binary alloys with some Se on the tie-lines GeTe--Sb.sub.2 Te.sub.3 and GeTe--Bi.sub.2 Te.sub.3 have higher sensitivity and quicker crystallizing rate. In U.S. Pat. No. 5,294,523, B or C of about 0 to 40 at. % is added to the alloy with Ge of 10 to 35 at. %, Sb of over 10 at. % and Te of 45 to 65 at. % to enhance the performance against repeated recording and erasing.
In Appl. Phys. Comm. 11(4) pp. 557 (1992) by Gonzalez-Hemandez etc., a new composition of Ge.sub.4 SbTe.sub.5 located along the GeTe--Sb.sub.2 Te.sub.3 tie-line was claimed and recognized as a single phase with FCC structure. However, J. H. Coombs found that the composition of the single phase alloy is Ge.sub.39 Sb.sub.9 Te.sub.52.
The change in reflectivity between the amorphous and the crystalline states of the recording film through the visible-wavelength range is of great important for phase change recording. Optical contrast denoted the ratio of the reflectance difference between the crystalline and the amorphous state to the reflectance of the crystalline state is a numerical index. The higher the optical contrast the larger the CNR is (Carrier to Noise Ratio) during reading. In the prior art, the optical contrast is of about 20 to 35%, and the CNR is of about 45 to 48 dB. Since the optical contrast of the prior art phase-change type recording media is low, the optical-disk players of the prior art must provide a better reading quality to compensate for the lower optical contrast. Consequently, the cost of the player of the prior art is increased.